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Behind the Red Knob

By Jeff PardoPublished Date

OK, which is it: Burned valves and detonation -- or better economy, lower maintenance costs, and greater range?OK, which is it: Burned valves and detonation -- or better economy, lower maintenance costs, and greater range? Like the paraphrased movie title of a different color, if there's anything that abducts attention (and opens up a can of worms in the process), it's leaning procedures.

WHAT is it, Anyway? Remember 'stoichiometry' from high school chemistry, balancing equations with the proper molar proportions of reactants, so every molecule gets to swing and do-si-do with a partner? As we go up and the air gets thinner, pulling that knob thins out the fuel accordingly. At some point, power and airspeed are maximized. Beyond that point (below peak cylinder head temperature and around peak exhaust gas temperature), fuel consumption is lowest. The gap between the two is typically small -- maybe 5% greater range with a 3% airspeed penalty. Lean further and you'll notice power loss and engine roughness.

Disclaimers: Given a choice between general engine manufacturer recommendations and your POH, the latter is the only one that takes into account the particular installation -- so it rules. Also, always return the mixture to rich before adding power. Butlean as aggressively as your POH allows. If you're doing it with panel gauges and not just the 'pull, listen, rough, enrich' method, remember that engine instruments are only as accurate as their calibrations.

Using an EGT: First, make sure that the adjustable needle is set for current conditions, not those of the previous flight! Lean for any changes in altitude, rpm, MP, or carb heat -- and do it slowly! Obviously, single-cylinder EGT probes must be on the meanest cylinder, but if you have an engine monitor, don't just lean to the hot cylinder at a *given* fuel flow, but the one that sizzles at the greatest fuel flow.

FINER POINTS: Folks who live up high frequently lean after start-up -- it prevents fouled plugs. If you're visiting a place that happens to be 'up high,' you should follow that example ... but, before you do, remember that the EGT is useless at idle. Proper leaning at altitude depends on the powerplant arrangement. Fixed-pitch folks, can lean to maximum rpm at full throttle before departure, when density altitude tops 5000 feet -- I sure hope your run-up area is free of gravel, though. Why 5000? ...because unless you're turbocharged, you won't top 75% power at 5000 feet. Both Lycoming and TCM use 75% as the 'OK' point for leaning out. Lycoming goes so far as to say that you can lean under 75% at any altitude. Again, refer to your POH for the best settings. Once airborne, use 'best power' to provide more even fuel distribution and the most speed for any given power setting.

METHODS FOR BEST POWER *Slowly* lean until the tach or airspeed peaks. (For the twisty-prop folks, use airspeed -- if you're not in smooth air, good luck.) 'Best economy' affords the most miles per gallon (again, for a given power setting).

TACH: Set power. Lean until the tach peaks. Airspeed will increase. At peak rpm, you're at max power. For best economy, lean rough, then enrich.

FUEL FLOW: Simple. Just lean by the marked flow values.

EGT method: Read the book, use what it says. The handbook for a '72 vintage Cherokee 235 that I fly says best power is 125 rich of peak, and best economy is 50.

Limits: Lycoming says you can lean to peak up to 75%; TCM says 50 degrees rich and right on is OK down at 65.

BOTTOM LINE: Leaning is a good way to improve the performance of your aircraft and the longevity of its innards, but it's not a cure-all to a bad atmospheric combo. Translation: If you're hell-bent on getting out of Dodge, leaning is not a justification to depart in nasty high-hot-humid conditions. Wait 'till it's cooler, and depart when the conditions return realistic safety factors to your takeoff planning numbers.

Pithy closing: Leaning. You can get worked up over it, or you can get educated.

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